The invention relates to an optical fiber based on silicon dioxide. The fiber has fluorine doping that determines its refractive index profile at least in the light transmitting part thereof, which part generally is called the core. The invention further relates to a method of manufacturing such an optical fiber in which a preform is produced by reactive deposition of silicon dioxide and fluorine from a reactive gas mixture. An optical fiber is manufactured from the preform. The processing of the preform into an optical fiber is usually done by a collapsing and/or drawing operation.
Optical fibers of the aforesaid kind are described in DE-OS No. 32 05 345 (corresponding to U.S. Pat. No. 4,468,413). Also known are fluorine-doped optical fibers in which the refractive index profile is determined not by the fluorine dopant but by other substances, while the fluorine dopant eliminates, for example, unwanted fluctuations in refractive index (DE-OS No. 30 31 147, corresponding to Canadian Pat. No. 1,162,948) or to give low OH-absorption (DE-OS No. 33 18 589).
It is further described in DE-AS No. 27 15 333 (corresponding to U.S. Pat. No. 4,062,665) that the thermal expansion coefficients of core and cladding of an optical fiber preform can be matched to one another so as to avoid crack formation.
Manufacturing methods of the aforementioned kind are for example the thermal deposition methods OVPO, MCVD and VAD, and also the PCVD method; cf DE-OS No. 32 05 345.
The exclusive use of fluorine-containing compounds as dopants for adjusting the refractive index profile offers the possibility of manufacturing optical fibers with various refractive index profiles in which the central region or core can largely consist of pure SiO.sub.2. Particular advantages thereby achieved are the avoidance of central "dips" (profile distortions originating during the collapsing operation), the low intrinsic scattering losses of SiO.sub.2 compared with those in other doping systems, the favorable material-dispersive properties of this system of monomode applications, and the comparatively low cost of fluorine-containing basic substances. In addition it is possible to easily obtain special profile structures, as for example "fully depressed" monomode structures (in which the refractive index distributions of the light transmitting core are below the refractive index of SiO.sub.2) and multimode structures with high numerical aperture.
In the investigations that resulted in the invention it was found that the advantageous optical properties achieved were subject to the following restrictions.
1. The material-intrinsic Rayleigh scattering losses of pure SiO.sub.2 were not experimentally attained, even with ideal light transmitting structures (SiO.sub.2 in the core, fluorine doped SiO.sub.2 in the cladding).
2. In fiber structures comprising only fluorine as the dopant the optical losses--compared with those in other doping systems--showed a distinct dependence on the drawing conditions. Rayleigh scattering and wavelength--independent scattering losses ("c term" losses) thereby depend in particular on the drawing force with which the fiber is drawn from the preform: high drawing forces result in low c-terms and increased Rayleigh scattering losses, whereas low drawing forces result as a rule in increased c-terms with at the same time low Rayleigh losses. This behavior raises problems for an absolute optimization of the drawing process; moreover the relative "optimum drawing conditions" for the pure fluorine-doping system differ distinctly from those of other doping systems (e.g. GeO.sub.2).
3. It was found experimentally that an increase in the proportion of fluorine-containing compounds in the reactive gas mixture to concentrations of more than 2 to 3 mol% (related to the chloride-gas proportions) does not result in any further reduction of OH-absorption losses; for high fluorine-dopant concentrations (maximum refractive index difference between core and cladding greater than the typical .DELTA.n.perspectiveto.1.5.times.10.sup.-2) it was observed that the OH-absorption losses were to some extent even higher than in fibers from low fluorine-dopant concentrations.
4. In spite of the somewhat comparable thermal expansion coefficients of pure SiO.sub.2 and fluorine-doped SiO.sub.2, cracks increased in the cladding of the collapsed preform.